Complex renal cysts (Bosniak ≥IIF): interobserver agreement, progression and malignancy rates.

• There is a good degree of agreement between radiologists in classifying complex renal masses using the Bosniak classification. • The rate of progression of Bosniak IIF cysts is low, and malignant cysts progress early during surveillance. Although the malignancy rates of resected Bosniak IIF, III and IV cysts are high, the rate of benign cyst resection is significant.